William is 3 times as old as Luis and is also 18 years older than Luis. How old is William?
Explanation: We can use the given information to write down two equations that describe the ages of William and Luis. Let William's current age be $w$ and Luis's current age be $l$ $w = 3l$ $w = l + 18$ Now we have two independent equations, and we can solve for our two unknowns. One way to solve for $w$ is to solve the second equation for $l$ and substitute that value into the first equation. Solving our second equation for $l$ , we get: $l = w - 18$ . Substituting this into our first equation, we get the equation: $w = 3$ $(w - 18)$ which combines the information about $w$ from both of our original equations. Simplifying the right side of this equation, we get: $w = 3w - 54$ Solving for $w$ , we get: $2 w = 54$ $w = 27$.